FIG. 4 shows a compensating circuit of a prior art high frequency amplifier which is disclosed in Japanese Published Patent Application 52-5240. In the figure, reference numeral 1 designates an input terminal, reference numeral 2 designates an output terminal, and reference numeral 12 designates a divider for dividing the input signal into three signals. Reference numeral 20 designates a distortion generating amplifier, reference numerals 21 and 22 designate linear amplifiers, and reference numerals 25a to 25d designate variable attenuators, and reference numeral 30a designates a combiner for combining the output of the distortion generating amplifier 20 with the output of the linear amplifier 21 in reversed phase to extract the distortion component. Reference numeral 30b designates a combiner for combining the distortion component which is extracted by the distortion extracting combiner 30a with the output of the linear amplifier 22 in reversed phase. Reference numeral 40 designates a high frequency amplifier the non-linearity of which is to be compensated.
A description is given of the operation of the circuit.
The signal which is input to the input terminal 1 is divided into three by the 12. The first divider output of the divider 12 is introduced to the distortion generating amplifier 20. The signal amplified by the distortion generating amplifier 20 is output and includes a distortion component. The output signal from amplifier 20 is introduced to the distortion extracting combiner 30a through the variable attenuator 25a. The second output of the divider 12 is introduced to the linear amplifier 21 through the variable attenuator 25b and amplified without a distortion component and is introduced to the distortion extracting combiner 30a. In the distortion extracting combiner 30a, the output including the distortion component of the distortion generating amplifier 20 and the output of the linear amplifier 21 including no distortion component are combined with the phase of one of the signals reversed so that only the distortion component of the output of the distortion generating amplifier 20 is extracted. That distortion component is introduced to the combiner 30b. The third output of the divider 12 is amplified without a distortion component by the linear amplifier 22 through the variable attenuator 25d and is supplied to the combiner 30b. At the combiner 30b, the output of the distortion extracting combiner 30a which has passed through the variable attenuator 25c, and the output of the linear amplifier 22 are combined with the phase of one of the outputs reversed. The combined output is input to the amplifier 40, the non-linearity of which is to be compensated, and the signal is amplified cancelling the distortion component generated by the amplifier 40. Thus, an output including no distortion component is output to the output terminal 2. Here, the variable attenuator 25c is adjusted such that it cancels the distortion component generated by the amplifier 40 and the distortion component is not output to the output terminal 2.
The compensating circuit of the prior art high frequency amplifier is constituted as described above. In order to correct the signal delay due to the distortion generating amplifier 25 a linear amplifier must be included in each of the other two signal paths, requiring two linear amplifiers, resulting in a large size circuit.
Since the number of dividers and combiners through which a signal passes is different depending on the signal path, it is not possible to keep the phase relation between the respective paths constant over a wide frequency range, making it impossible to compensate for the non-linearity of the high frequency amplifier over a wide frequency range.